The presence of surveillance mechanisms have been postulated to be the basis by which the host inhibits the development of tumors from induced mutagenic malignant changes in somatic cell populations. A major factor in the regulation of host environment, as related to foreignness of a variety of viable and inert agents is the reticuloendothelial system. Functional expression of the reticuloendothelial system requires not only normal macrophage cellular function but also, as we have demonstrated in previous studies, the presence of opsonins or recognition factors. The latter are essential for the expression of the "self" - "non-self" discriminative actions of macrophages. In view of our previous studies relative to the loss of recognition factor activity in patients with neoplasia and the relationship between degree of depletion of recognition factor activity and the clinical state of the subject, our proposed studies will continue to clarify the relative importance of recognition factors and macrophages in clinical and experimental neoplasia. The role of recognition factors in the inhibition of tumor growth will continue to be established, as will the role of recognition factors as chemotactic agents. The possible therapeutic role of recognition factors will be ascertained in studies in rats with transplanted leukemia. The appreciation of the role of recognition factors in the identification of tumor cells in the environment and in the possible destruction of tumor cells by macrophages will contribute to the further appreciation of the possible decisive role of macrophages in neoplasia.